The U.S. egg industry produced more than 300 million pounds of frozen pasteurized egg products in 1985. The relatively mild pasteurization processes used are designed to eliminate Salmonella from eggs for all practical purposes, but do not destroy organisms capable of spoiling egg products held under refrigerated temperatures above freezing. The survival of spoilage organisms in pasteurized egg, coupled with conventional packaging technology, results in products with limited shelf life (7-14 days at 40.degree. F.) that require freezing and frozen distribution systems for preservation. Freezing is cost intensive, lowers the functional quality (flavor, performance, etc.) of the egg product, and results in an inconvenient product that requires thawing prior to use.
Standard pasteurization practices for liquid whole egg products are generally discussed in the Egg Pasteurization Manual (USDA Agricultural Research Service 1969). See also U.S. Pat. No. 3,949,099 to Kaufman, at column 1, lines 46-58 (summarizing pasteurization processes). The Egg Pasteurization Manual notes that experiments conducted in England to establish treatment conditions used there, which were carried out in a high temperature, short time pasteurization unit, included trials with holding times of two and one-half minutes and temperatures up to 151 degrees Fahrenheit (66.1 degrees Centigrade). The functional quality of these eggs is not discussed; however, it is noted that England adopted a lower temperature of 148 degrees Fahrenheit as a treatment condition. The Manual observes that such "severe" treatments help increase the stability of egg products in liquid form, but does not teach how this can be accomplished without sacrificing desirable functional properties of the products. For other types of egg products which are more heat stable, pasteurization temperatures as high as 152 degrees Fahrenheit (66.67 degrees Centigrade) have been reported. Again, little is known about how to ultrapasteurize (decrease the number of spoilage microorganisms to levels lower than obtained with a pasteurization procedure) these products by heating without sacrificing functional performance.
Higher temperatures than those investigated in England have been explored. See e.g., Essary, E. O., Rony, P. R., and Collins, W. F., New uses of heated aseptically packaged fluid egg products, Report to the American Egg Board (1983). This work suggested that merging techniques of ultrapasteurization with aseptic packaging can be accomplished to yield safe, stable, and functional liquid whole egg. This research suggested and used a heat exchanger operated in laminar flow to process whole egg at elevated temperatures, and used an ethylene oxide-flushed glove box to aseptically package the product (neither device approved for the commercial production of a salable product). Holding temperatures of up to 154 degrees Fahrenheit (67.8 degrees Centigrade) were reported. The results of this pioneering work demonstrated the possibility of extending the refrigerated shelf life of liquid whole egg products, but did not enable the commercial production of a refrigerated product which could be sold to consumers.
To carry out ultrapasteurization of liquid whole egg products on commercial equipment, the time and temperature parameters of the thermal treatment must be selected with care. Arbitrary increases in thermal treatment will likely produce a functionally unacceptable product, or--even worse--coagulation of the product and blockage of the pasteurizing equipment. Because such pasteurizing equipment operates at high pressures (e.g., 1,500 p.s.i. and more) the consequences of blockage are serious. For these reasons, it is essential to know how liquid whole egg products respond to various thermal treatments before experimenting with the production of these products on commercial equipment.
Because this essential information was not available, Hamid-Samimi investigated the time and temperature parameters which should be used in commercial equipment to obtain a functionally acceptable, ultrapasteurized, liquid whole egg product. M. H. Hamid-Samimi, Criteria Development for Extended Shelf-Life Pasteurized Liquid Whole Egg. Ph.D. Thesis, North Carolina State University (1984). This research was carried out at the laboratory bench, with a Brookfield viscometer, which processed the product in small batches rather than continuously. Processing times and temperatures defining thermal treatments producing a soluble protein loss (SPL) of up to 5% from the product, as an upper limit, were suggested as producing a functionally acceptable product. The suggested 5% SPL limit was defined by a graph line in the thesis: this line is reproduced herein in FIG. 3 as the 5% SPL (Batch) line. Time and temperature values tested by other investigators which were believed to be the limits of pasteurization are summarized in M. H. Hamid-Samimi and K. R. Swartzel, J. Food Processing and Preservation 8: 219, 221 FIG. 1 (1984). All are below the 5% SPL (Batch) line.
The present inventors sought to test the predictions of Hamid-Samimi by ultrapasteurizing liquid whole egg on continuous flow, high temperature, short time pasteurization equipment. It was unexpectedly found that functionally acceptable liquid whole egg can be produced at times and temperatures greater than previously believed. These findings, which will be explained in detail below, enable the commercial production of functionally acceptable products with longer refrigerated shelf lives than heretofore available. As will also be explained below, discovery of these unexpected results has led to the identification of several steps which should be taken to produce ultrapasteurized liquid whole egg products that have superior functional properties.
The object of the present invention, in short, is to provide liquid whole egg products for refrigerated distribution which have greatly reduced levels of spoilage microorganisms, while still having good functional properties.